A dynamical analysis of the dust tail of Comet C/1995 O1 (Hale–Bopp) at high heliocentric distances

• Hale–Bopp was active at the large heliocentric distances of 21.6 and 27.2 AU.
• The structure of the tail changed significantly in 3.3 years between observations.
• Commonly used dynamical models cannot explain the shape of the tail.
• The Lorentz force likely plays a major role in the motion of the dust in the tail.
• The tail is likely made of β = 0.3 particles, with radii of ∼0.5–5 μm.

Comet C/1995 O1 (Hale–Bopp) has provided an unprecedented opportunity to observe a bright comet over a wide range of heliocentric distances. We present here Spitzer Space Telescope observations of Hale–Bopp from 2005 to 2008 that show a distinct coma and tail, the presence of which is uncommon given its large heliocentric distance (21.6 AU and 27.2 AU, respectively). The morphology of the dust is compared to dynamical models to understand the activity of the comet. Our analysis shows that the shape of Hale–Bopp’s dust tail in these images cannot be explained using the usual Finson–Probstein (solar gravity + solar radiation pressure) dynamical model. Several alternative explanations are explored. The analysis suggests that the most likely cause of the discrepancy is that the dust is being charged by the solar wind, then being affected by the interplanetary magnetic field via the Lorentz force. Though this effect has been explored previously, if correct, this seems to be the first time that the Lorentz force has been required to model a cometary dust tail. The analysis also suggests that Hale–Bopp was actively emitting particles when these images were taken, and the tail characteristics changed between observations.